An example control scheme for an omnidirectional condenser microphone is an elastic control scheme. In this scheme, the resonance frequency of the mechanical vibration system of a non-directional condenser microphone is set to be a high frequency close to the upper limit of the sound collection band. As a result, a frequency response of the omnidirectional condenser microphone in a frequency band lower than or equal to the resonance frequency becomes flat.
When a resonance frequency of the condenser microphone is set outside the audible range, a frequency response in the entire sound collection band becomes flat and the sensitivity of the condenser microphone decreases. On the other hand, when a resonance frequency of the condenser microphone is set near the middle of the sound collection band, the sensitivity of the condenser microphone increases and the frequency response decreases with a slope of −12 dB/Oct in a frequency band higher than or equal to the resonance frequency. Thus, by setting the resonance frequency close to the upper limit (approximately 10 kHz) of the sound collection band and then adjusting the resonance sharpness, the resonance response in the sound collection band of the condenser microphone is flattened.
FIG. 5 is a cross-sectional side view illustrating a conventional omnidirectional condenser microphone.
A condenser microphone unit (hereinafter referred to as “conventional unit”) 2a includes a unit case 2c and an electroacoustic transducer 20. The electroacoustic transducer 20 converts acoustic waves from a sound source to electrical signals and outputs the electrical signals. The electroacoustic transducer 20 is accommodated in the unit case 2c. The conventional unit 2a is attached to a circuit case (not shown).
The unit case 2c is composed of metal. The unit case 2c has a shape of a hollow cylinder with a closed end. A bottom face of the unit case 2c is disposed at the front (the direction of the microphone that is directed to the sound source during sound collection, the same applies hereinafter) side of the unit case 2c. The unit case 2c includes an acoustic-wave entering hole 2h, an open end 2e, a flange 2f, and an internal thread 2s. The acoustic-wave entering hole 2h introduces acoustic waves from a sound source into the unit case 2c. The acoustic-wave entering hole 2h is disposed in the bottom face of the unit case 2c. The open end 2e is the rear end of the unit case 2c. The flange 2f is composed of the bottom face of the unit case 2c having the acoustic-wave entering hole 2h. The internal thread 2s corresponds to an external thread provided on the circuit case (not shown). The internal thread 2s is disposed at the rear side of the inner circumferential surface of the unit case 2c. 
The electroacoustic transducer 20 includes a diaphragm holder (diaphragm ring) 21, a diaphragm 22, a spacer 23, a fixed electrode 24, an insulator 25, a support 26, an insulating base 27, an electrode extraction terminal 28, and a contact pin 29.
The diaphragm holder 21 supports the diaphragm 22. The diaphragm holder 21 is ring-shaped. The diaphragm holder 21 has a hole in its center. The diaphragm 22 has a shape of a disc. The diaphragm 22 has a metal (preferably gold) film deposited on one side. The diaphragm 22 is a thin film composed of synthetic resin. The diaphragm 22 is stretched on the diaphragm holder 21 with predetermined tension. The spacer 23 is composed of synthetic resin, for example. The spacer 23 has a shape of a thin ring. The fixed electrode 24 is composed of metal. The fixed electrode 24 has a shape of a disc. At least one of the faces of the fixed electrode 24, for example, the face adjacent to the diaphragm 22, has an electret plate bonded thereto. The fixed electrode 24 and the electret plate constitute an electret board. The diaphragm 22 is disposed adjacent to the fixed electrode 24 with the spacer 23. A layer of air (gap) having a thickness equivalent to that of the spacer 23 is positioned between the diaphragm 22 and the fixed electrode 24. The diaphragm 22 and the fixed electrode 24 constitute a capacitor. The capacitance of the capacitor varies with the vibration of the diaphragm 22 in response to acoustic waves from a sound source, passing through the acoustic-wave entering hole 2h. 
The insulator 25 supports the fixed electrode 24 and electrically insulates the fixed electrode 24 from the unit case 2c and the diaphragm 22. The insulator 25 has multiple communication holes. The penetrating direction of the communication holes is the thickness direction (the horizontal direction in FIG. 5) of the insulator 25.
The support 26 is attached to the rear face of the insulator 25 in an airtight manner. Air chambers are defined between the fixed electrode 24 and the insulator 25 and between the insulator 25 and the support 26 via the communication holes of the insulator 25.
The insulating base 27 is disposed behind the support 26. The insulating base 27 has a connection hole. The penetrating direction of the connection hole is the thickness direction (the horizontal direction in FIG. 5) of the insulating base 27.
The electrode extraction terminal 28 extracts signals from the fixed electrode 24. The electrode extraction terminal 28 is attached to the central area of the insulator 25. The rear end portion of the electrode extraction terminal 28 is inserted into the front half of the connection hole of the insulating base 27. The contact pin 29 is electrically connected to the electrode extraction terminal 28 via an elastic material (not shown) such as a conductive sponge. The contact pin 29 is inserted into the rear half of the connection hole of the insulating base 27.
The electroacoustic transducer 20 is fixed inside the unit case 2c with a lock ring 20r that fits the internal thread 2s. 
A field effect transistor (FET) and a circuit, for example, are included in the circuit case. The FET constitutes an impedance converter of the electroacoustic transducer 20. The circuit is, for example, a circuit which converts a variation in the capacitance between the diaphragm 22 and the fixed electrode 24 to electrical signals and outputs the electrical signals.
FIG. 6 illustrates an equivalent circuit of a conventional omnidirectional condenser microphone.
In FIG. 6, symbol p represents the sound pressure of acoustic waves from a sound source; symbol m0 represents the mass of the diaphragm 22; symbol s0 represents the stiffness of the diaphragm 22; symbol r0 represents the damping resistance of the diaphragm 22 due to the layer of air between the diaphragm 22 and the fixed electrode 24; symbol rf represents the acoustic resistance in front of the diaphragm 22 (at the front open end among the front and rear open ends of the hole of the ring diaphragm holder 21, the front open end facing the rear open end which the diaphragm 22 is stretched on); symbol sf represents the stiffness of the air chamber (the internal space in the hole in the ring diaphragm holder 21) at the front of the diaphragm 22; and symbol s1 represents the stiffness of the air chamber at the rear of the diaphragm 22.
The damping resistance r0 of the diaphragm 22 reduces the resonance sharpness to a certain degree. However, by the shape effect, the frequency response in a frequency band higher than or equal to the resonance frequency increases. Thus, the adjustment of the frequency response by adding an acoustic resistor to the front of the diaphragm 22 is required. Schemes have been proposed to make acoustic resistance of an acoustic resistor disposed at the front of a diaphragm variable to adjust the frequency response (for example, refer to Japanese Unexamined Patent Application Publication No. 2000-50386).